Inverting Amplifier Configuration

Figure 3 shows a single channel of the MAX9727 con-

digitally controlled potentiometer to alter the gain under

software control.

Active Filter Configuration

When the MAX9727 is used as a line driver to provide

outputs that feed audio equipment (notebooks, desk-

tops, receivers, and set-top boxes) with a digital-to-

analog converter (DAC) used as an audio input source,

it is often desirable to eliminate any high-frequency

quantization noise produced by the DAC output before

it reaches the load. This high-frequency noise can

cause the input stages of the line-in equipment to

exceed slew-rate limitations or create excessive EMI

emissions on the cables between devices.

standard audio output level from a single 5V supply,

the MAX9727 can be configured as an active lowpass

filter. The

Functional Diagram/Typical Application

Circuit shows the MAX9727 connected as 2-pole

Rauch/Multiple Feedback filter with a passband gain of

6dB and a -3dB (below passband) cutoff frequency of

approximately 27kHz (see Figure 4 for Gain vs.

Frequency plot).

Input Filter

a highpass filter that removes the DC bias from an

incoming signal. The AC-coupling capacitor allows the

amplifier to bias the signal to an optimum DC level.

Assuming zero-source impedance, the -3dB point of

the highpass filter is given by:

response of the amplifier. Use capacitors with

dielectrics that have low-voltage coefficients, such as

tantalum or aluminum electrolytic. Capacitors with high-

voltage coefficients, such as ceramics, can increase

distortion at low frequencies.

Charge-Pump Capacitor Selection

Use capacitors with an ESR of less than 100mΩ for

optimum performance. Low-ESR ceramic capacitors

minimize the output resistance of the charge pump. For

best performance over the extended temperature

range, select capacitors with an X7R dielectric.

f

RC

dB

IN IN

−

=

3

1

2

π

A

R

R

V

F

IN

10

______________________________________________________________________________________

IN_-

IN_+

OUT_

MAX9727

Figure 3. Inverting Amplifier Configuration

ACTIVE FILTER GAIN vs. FREQUENCY

FREQUENCY (kHz)

10

-12

-9

-6

-3

0

3

6

9

-15

1

100

Figure 4. MAX9727 Active Filter Gain vs. Frequency